No xp boot up since today's update

I have an XP pc that I recently upgraded from an ATI 3850 to an ATI 4650 graphics card. Since this upgrade the pc would occasionally not boot up at all and after throwing the power off and on it would boot up again, but earlier this morning the security center notified me of a new XP update which I did. I shut the pc down an hour later and now this evening I can't get it to boot up at all. Nothing but a blank black screen!